Maxim Integrated High-Speed Microcontroller Users Guide: Network Microcontroller Supplement - Manual (Page 95)

ADDENDUM TO SECTION 5: CPU TIMING
External Clock Source
The DS80C400 supports a maximum operating frequency of 75MHz. However, when using an external crystal, the frequency must not
exceed 40MHz in order for the internal oscillator circuitry to work properly. Thus, the maximum operating frequency can be achieved
in one of two ways: 1) use of a stand-alone clock oscillator or clock source (up to 75MHz) to directly drive the XTAL1 pin or 2) use of
the on-chip clock multiplier circuitry (described later) to 4X/2X multiply the external crystal frequency.
System Clock Selection
The internal clocking options of the DS80C400 differ slightly from that described in the High-Speed Microcontroller User’s Guide. Most
members of the family offer the option of 4, 256, or 1024 clocks per machine cycle. The DS80C400 can operate at 1, 2, 4, or 1024
oscillator clocks per machine cycle. The system clock divide control function is shown in Figure 5-1. A 3:1 multiplexer, controlled by
CD1, CD0 (PMR.7-6), selects one of three sources for the internal system clock:
• Crystal oscillator or external clock source
• Crystal oscillator or external clock source divided by 256
• Crystal oscillator or external clock source frequency multiplied by 2 or 4
The system clock control circuitry generates two clock signals that are used by the microcontroller. The internal system clock provides
the time base for timers and internal peripherals. The system clock is run through a divide-by-4 circuit to generate the machine cycle
clock that provides the time base for CPU operations. All instructions execute in one to six machine cycles. It is important to note the
distinction between these two clock signals as they are sometimes confused, creating errors in timing calculations.
Setting CD1:0 to 00b enables the frequency multiplier, either doubling or quadrupling the frequency of the crystal oscillator or external
clock source. The 4X/2X bit controls the multiplying factor, selecting two or four times the frequency when set to 0 or 1, respectively.
Enabling the frequency multiplier results in apparent instruction execution speeds of 2 or 1 oscillator clocks. Regardless of the con-
figuration of the frequency multiplier, the system clock of the microcontroller can never be operated faster than 75MHz. This means
that the maximum crystal oscillator or external clock source is 18.75MHz when using the 4X setting and 37.5MHz when using the 2X
setting.
The primary advantage of the clock multiplier is that it allows the microcontroller to use slower crystals to achieve the same perfor-
mance level. This reduces EMI and cost, as slower crystals are generally more available and, therefore, less expensive.
High-Speed Microcontroller User’s
Guide: Network Microcontroller
Supplement
95
Figure 5-1. System Clock Control Diagram
Maxim Integrated